July 16, 2009

Materialized Views

Materialized views in Oracle

Oracle materialized views were first introduced in Oracle8.

Materialized views are schema objects that can be used to summarize, precompute, replicate and distribute data.

In mview, the query result is cached as a concrete table that may be updated from the original base tables from time to time. This enables much more efficient access, at the cost of some data being potentially out-of-date. It is most useful in datawarehousing scenarios, where frequent queries of the actual base tables can be extremely expensive.

Oracle uses materialized views (also known as snapshots in prior releases) to replicate data to non-master sites in a replication environment and to cache expensive queries in a datawarehouse environment.

A materialized view is a database object that contains the results of a query. They are local copies of data located remotely, or are used to create summary tables based on aggregations of a table's data.

A materialized view is a replica of a target master from a single point in time. We can define a materialized view on a base/master table (at a master site), partitioned table, view, synonym or a master materialized view (at a materialized view site). Whereas in multi master replication tables are continuously updated by other master sites, materialized views are updated from one or more masters through individual batch updates, known as a refreshes, from a single master site or master materialized view site.

A materialized view provides indirect access to table data by storing the results of a query in a separate schema object. Unlike an ordinary view, which does not take up any storage space or contain any data, Mview stores data, whereas view stores only query.

The existence of a materialized view is transparent to SQL, but when used for query rewrites will improve the performance of SQL execution. An updatable materialized view lets you insert, update, and delete.

We can define indexes on a materialized view.

A materialized view can be stored in the same database as it's base table(s) or in a different database. Materialized views stored in the same database as their base tables can improve query performance through query rewrites. Query rewrites are particularly useful in a datawarehouse environment.

A materialized view can query tables, views and other materialized views. Collectively these are called master tables (a replication term) or detail tables (a datawarehouse term).

For replication purposes, materialized views allow us to maintain copies of remote data on local node. These copies are read-only. If we want to update the local copies, we have to use the Advanced Replication feature. We can select data from a materialized view as we would from a table or view.

Whenever you create a materialized view, regardless of it's type, always specify the schema name of the table owner in the query for the materialized view.

Prerequisites:

To create mviews, the user should have any one of

CREATE MATERIALIZED VIEW or CREATE ANY MATERIALIZED VIEW privileges.

SQL> GRANT CREATE MATERIALIZED VIEW TO user-name;

And

SQL> GRANT QUERY REWRITE TO user-name;

And following init parameters should be set

query_rewrite_enabled = true (default)

query_rewrite_integrity = enforced|trusted|stale_tolerated

The background processes responsible for these materialized view refreshes are the coordinated job queue (CJQ) processes.

job_queue_processes=n

Syntax:

CREATE MATERIALIZED VIEW mview-name

[partitioning-options]

[storage-parameters]

[TABLESPACE tablespace-name]

[OF object-type]

[FOR UPDATE]

[BUILD IMMEDIATE|BUILD DEFFERED|ON PREBUILT TABLE]

[REFRESH [FAST|COMPLETE|FORCE|NEVER]

[ON DEMAND|COMMIT]

[START WITH date]

[NEXT date]

[WITH PRIMARY KEY|ROWID]]

[DISABLE|ENABLE QUERY REWRITE]

AS select-query;

Refresh Types

Oracle can refresh a materialized view using either a fast, complete or force refresh.

The refresh option specifies:

a. Refresh method used by Oracle to refresh data in materialized view. FORCE is the default option.

b.Whether the view is primary key based or row-id based. PRIMARY KEY is the default option.

c. Time and interval at which the view is to be refreshed.

Complete Refresh

To perform COMPLETE refresh of a materialized view, the server that manages the materialized view executes the materialized view's defining query, which essentially recreates the materialized view. To refresh the materialized view, the result set of the query replaces the existing materialized view data. Oracle can perform a complete refresh for any materialized view. Depending on the amount of data that satisfies the defining query, a complete refresh can take a substantially longer amount of time to perform than a fast refresh.

Note: If a materialized view is complete refreshed, then set it's PCTFREE to 0 and PCTUSED to 99 for maximum efficiency.

The complete refresh re-creates the entire materialized view. If we request a complete refresh, Oracle performs a complete refresh even if a fast refresh is possible.

SQL> CREATE MATERIALIZED VIEW mv_emp

REFRESH COMPLETE

START WITH SYSDATE NEXT SYSDATE + 1

WITH PRIMARY KEY

AS SELECT * FROM emp@remote_db;

To refresh this mview,

SQL> EXEC DBMS_MVIEW.REFRESH('mv_emp', 'C');

From Oracle 10g, complete refresh of single materialized view can do delete instead of truncate. To force the refresh to do truncate instead of delete, parameter ATOMIC_REFRESH must be set to false.

ATOMIC_REFRESH = FALSE, mview will be truncated and whole data will be inserted. The refresh will go faster, and no undo will be generated.

ATOMIC_REFRESH = TRUE (default), mview will be deleted and whole data will be inserted. Undo will be generated. We will have access at all times even while it is being refreshed.

SQL> EXEC DBMS_MVIEW.REFRESH('mv_emp', 'C', atomic_refresh=>FALSE);

If we perform complete refresh of a master materialized view, then the next refresh performed on any materialized views based on this master materialized view must be a complete refresh. If a fast refresh is attempted for such a materialized view after it's master materialized view has performed a complete refresh, then Oracle returns the following error:

ORA-12034 mview log is younger than last refresh

Fast Refresh

To perform FAST refresh, the master that manages the materialized view first identifies the changes that occurred in the master since the most recent refresh of the materialized view and then applies these changes to the materialized view. Fast refreshes are more efficient than complete refreshes when there are few changes to the master because the participating server and network replicate a smaller amount of data.

We can perform fast refreshes of materialized views only when the master table or master materialized view has a materialized view log. Also, for fast refreshes to be faster than complete refreshes, each join column in the CREATE MATERIALIZED VIEW statement must have an index on it.

SQL> CREATE MATERIALIZED VIEW mv_emp

BUILD IMMEDIATE

REFRESH FAST

START WITH SYSDATE NEXT SYSDATE + 2

WITH PRIMARY KEY

ENABLE QUERY REWRITE

AS SELECT * FROM emp@remote_db;

A materialized view logis a schema object that records changes to a master table's data so that a materialized view defined on the master table can be refreshed incrementally.

We should create a materialized view log for the master tables if we specify the REFRESH FAST clause.

SQL> CREATE MATERIALIZED VIEW LOG ON emp;

To refresh this mview,

SQL> EXEC DBMS_MVIEW.REFRESH('mv_emp', 'F');

After a direct path load on a master table or master materialized view using SQL*Loader, a fast refresh does not apply the changes that occurred during the direct path load. Also, fast refresh does not apply changes that result from other types of bulk load operations on masters. Examples of these operations include some INSERT statements with an APPEND hint and some INSERT ... SELECT * FROM statements.

Note:

->> Fast refreshable materialized views can be created based on master tables and master materialized views only.

->> Materialized views based on a synonym or a view must be complete refreshed.

->> Materialized views are not eligible for fast refresh if the defined subquery contains an analytic function.

Force Refresh

To perform FORCE refresh of a materialized view, the server that manages the materialized view attempts to perform a fast refresh. If fast refresh is not possible, then Oracle performs complete refresh. Use the force setting when you want a materialized view to refresh if fast refresh is not possible.

If you do not specify a refresh method, FORCE is the default.

SQL> CREATE MATERIALIZED VIEW mv_emp

REFRESH FORCE

START WITH SYSDATE NEXT SYSDATE + 3

WITH PRIMARY KEY

DISABLE QUERY REWRITE

AS SELECT * FROM emp@remote_db;

To refresh this mview,

SQL> EXEC DBMS_MVIEW.REFRESH(LIST =>'mv_emp', METHOD =>'?');

(or)

SQL> EXEC DBMS_MVIEW.REFRESH(LIST =>'mv_emp');

Partition Change Tracking (PCT) Refresh

In Oracle9i, a new mechanism called Partition Change Tracking (PCT) has been introduced. This mechanism keeps track of the base table partitions that have been updated since the materialized view was last refreshed. It tracks partition maintenance operations (PMOPs), like add and truncate partition as well as DML changes to the base data, on partition basis. This allows Oracle to identify fresh data in the materialized view.

Partition Change Tracking (PCT) refresh refers to MV refresh using only the changed partitions of the base tables of an MV. This refresh method is possible only if the base tables are partitioned and changes to base tables are tracked on a partition basis.

Enhanced Partition Change Tracking (EPCT) refresh refers to PCT based refresh applied to MVs containing columns that are partition-join dependent on the partitioning column of the base table.

Timing the refresh

The START WITH clause tells the database when to perform the first replication from the master table to the local base table. It should evaluate to a future point in time. The NEXT clause specifies the interval between refreshes

SQL> CREATE MATERIALIZED VIEW mv_emp_pk

REFRESH FAST

START WITH SYSDATE NEXT SYSDATE + 2

WITH PRIMARY KEY

AS SELECT * FROM emp@remote_db;

In the above example, the first copy of the materialized view is made at SYSDATE (immediately) and the interval at which the refresh has to be performed is every two days.

SQL> CREATE MATERIALIZED VIEW mv_emp_pk

REFRESH COMPLETE

START WITH SYSDATE NEXT SYSDATE + 2/(24*60)

WITH ROWID

AS SELECT * FROM emp@remote_db;

In this example, the interval is two minutes. For every two minutes, fast refresh will happen.

SQL> CREATE MATERIALIZED VIEW mv_emp_pk

REFRESH FORCE

START WITH SYSDATE NEXT SYSDATE + 30/(24*60*60)

WITH PRIMARY KEY

AS SELECT * FROM emp@remote_db;

In this example, the interval is 30 seconds.

SQL> CREATE MATERIALIZED VIEW mv_emp_f

REFRESH FAST ON COMMIT

WITH PRIMARY KEY

AS SELECT * FROM emp@remote_db;

SQL> CREATE MATERIALIZED VIEW mv_emp_c

REFRESH COMPLETE ON DEMAND

WITH ROWID

AS SELECT * FROM emp@remote_db;

SQL> EXECUTE DBMS_MVIEW.REFRESH('mv_emp_c','C');

PRIMARY KEY and ROWID Clause

WITH PRIMARY KEY is used to create a primary key materialized view i.e. the materialized view is based on the primary key of the master table instead of ROWID. PRIMARY KEY is the default option. To use the PRIMARY KEY clause we should have defined PRIMARY KEY on the master table or else you should use ROWID based materialized views.

A materialized view can be either read-only, updatable or writeable. Users cannot perform data manipulation language (DML) statements on read-only materialized views, but they can perform DML on updatable and writeable materialized views.

1. Read only

Cannot be updated and complex materialized views are supported.

2. Updateable

Can be updated even when disconnected from the master site.

Are refreshed on demand.

Consumes fewer resources.

Requires Advanced Replication option to be installed.

3. Writeable

Created with the for update clause.

Changes are lost when view is refreshed.

Requires Advanced Replication option to be installed.

Note: For read-only, updatable, and writeable materialized views, the defining query of the materialized view must reference all of the primary key columns in the master.

Read-Only Materialized Views

We can make a materialized view read-only during creation by omitting the FOR UPDATE clause or disabling the equivalent option in the Replication Management tool. Read-only materialized views use many of the same mechanisms as updatable materialized views, except that they do not need to belong to a materialized view group.

In addition, using read-only materialized views eliminates the possibility of a materialized view introducing data conflicts at the master site or master materialized view site, although this convenience means that updates cannot be made at the remote materialized view site.

The following is an example of a read-only materialized view:

SQL> CREATE MATERIALIZED VIEW hr.emp

AS SELECT * FROM hr.emp@orcl;

Updatable Materialized Views

We can make a materialized view updatable during creation by including the FOR UPDATE clause or enabling the equivalent option in the Replication Management tool. For changes made to an updatable materialized view to be pushed back to the master during refresh, the updatable materialized view must belong to a materialized view group.

Updatable materialized views enable us to decrease the load on master sites because users can make changes to the data at the materialized view site.

The following is an example of an updatable materialized view:

SQL> CREATE MATERIALIZED VIEW hr.dept

FOR UPDATE

AS SELECT * FROM hr.departments@orcl;

The following statement creates a materialized view group:

BEGIN

DBMS_REPCAT.CREATE_MVIEW_REPGROUP (

gname => 'hr_repg',

master => 'orcl',

propagation_mode => 'ASYNCHRONOUS');

END;

/

The following statement adds the hr.departments materialized view to the materialized view group, making the materialized view updatable:

BEGIN

DBMS_REPCAT.CREATE_MVIEW_REPOBJECT (

gname => 'hr_repg',

sname => 'hr',

oname => 'departments',

type => 'SNAPSHOT',

min_communication => TRUE);

END;

/

We can also use the Replication Management tool to create a materialized view group and add a materialized view to it.

Note:

Do not use column aliases when we are creating an updatable materialized view. Column aliases cause an error when we attempt to add the materialized view to a materialized view group using the CREATE_MVIEW_REPOBJECT procedure.

An updatable materialized view based on a master table or master materialized view that has defined column default values does not automatically use the master's default values.

Updatable materialized views do not support the DELETE CASCADE constraint.

The following types of materialized views cannot be masters for updatable materialized views:

ROWID materialized views

Complex materialized views

Read-only materialized views

However, these types of materialized views can be masters for read-only materialized views.

Additional Restrictions for Updatable Materialized Views Based on Materialized Views, those must:

Belong to a materialized view group that has the same name as the materialized view group at it's master materialized view site.

Reside in a different database than the materialized view group at it's master materialized view site.

Be based on another updatable materialized view or other updatable materialized views, not on a read-only materialized view.

Be based on a materialized view in a materialized view group that is owned by PUBLIC at the master materialized view site.

Writeable Materialized Views

A writeable materialized view is one that is created using the FOR UPDATE clause but is not part of a materialized view group. Users can perform DML operations on a writeable materialized view, but if we refresh the materialized view, then these changes are not pushed back to the master and the changes are lost in the materialized view itself. Writeable materialized views are typically allowed wherever fast-refreshable read-only materialized views are allowed.

We can use materialized views to achieve one or more of the following goals:

Less network loads

Create mass deployment environment

Enable data subsetting

Enable disconnected computing

Less Network Loads

We can use materialized views to reduce network loads & to distribute your corporate database to regional sites. Instead of the entire company accessing a single database server, user load is distributed across multiple database servers. Through the use of multitier materialized views, we can create materialized views based on other materialized views, which enables us to distribute user load to an even greater extent because clients can access materialized view sites instead of master sites. To decrease the amount of data that is replicated, a materialized view can be a subset of a master table or master materialized view.

While multimaster replication also distributes a database among multiple sites, the networking requirements for multimaster replication are greater than those for replicating with materialized views because of the transaction by transaction nature of multimaster replication. Further, the ability of multimaster replication to provide real-time or near real-time replication may result in greater network traffic, and might require a dedicated network link.

Materialized views are updated through an efficient batch process from a single master site or master materialized view site. They have lower network requirements and dependencies than multimaster replication because of the point in time nature of materialized view replication. Whereas multimaster replication requires constant communication over the network, materialized view replication requires only periodic refreshes.

In addition to not requiring a dedicated network connection, replicating data with materialized views increases data availability by providing local access to the target data. These benefits, combined with mass deployment and data subsetting (both of which also reduce network loads), greatly enhance the performance and reliability of your replicated database.

Create mass deployment environment

Deployment templates allow us to precreate a materialized view environment locally. We can then use deployment templates to quickly and easily deploy materialized view environments to support sales force automation and other mass deployment environments. Parameters allow us to create custom data sets for individual users without changing the deployment template. This technology enables you to roll out a database infrastructure to hundreds or thousands of users.

Enable data subsetting

Materialized views allow us to replicate data based on column and row-level subsetting, while multimaster replication requires replication of the entire table. Data subsetting enables us to replicate information that pertains only to a particular site. For example, if we have a regional sales office, then we might replicate only the data that is needed in that region, thereby cutting down on unnecessary network traffic.

Enable disconnected computing

Materialized views do not require a dedicated network connection. Though we have the option of automating the refresh process by scheduling a job, we can manually refresh your materialized view on-demand, which is an ideal solution for sales applications running on a laptop. For example, a developer can integrate the replication management API for refresh on-demand into the sales application. When the salesperson has completed the day's orders, the salesperson simply dials up the network and uses the integrated mechanism to refresh the database, thus transferring the orders to the main office.

Note:

Both the master site and the materialized view site must have compatibility level (COMPATIBLE initialization parameter) 9.0.1 or higher to replicate user-defined types and any objects on which they are based.

We cannot create refresh-on-commit materialized views based on a master with user-defined types. Refresh-on-commit materialized views are those created using the ON COMMIT REFRESH clause in the CREATE MATERIALIZED VIEW statement.

A materialized view group in a replication system maintains a partial or complete copy of the objects at the target replication group at it's master site or master materialized view site. Materialized view groups cannot span the boundaries of the replication group at the master site or master materialized view site.

Group A at the materialized view site contains only some of the objects in the corresponding Group A at the master site. Group B at the materialized view site contains all objects in Group B at the master site. Under no circumstances, however, could Group B at the materialized view site contain objects from Group A at the master site. A materialized view group has the same name as the master group on which the materialized view group is based. For example, a materialized view group based on a personnel master group is also named personnel.

In addition to maintaining organizational consistency between materialized view sites and their master sites or master materialized view sites, materialized view groups are required for supporting updatable materialized views. If a materialized view does not belong to a materialized view group, then it must be a read-only or writeable materialized view.

Managing MVs is much easier in Oracle 10g with the introduction of the powerful new tuning advisors that can tell us a lot about the design of the MVs. Tuning recommendations that can generate a complete script that can be implemented quickly, saving significant time and effort. The ability to force rewriting or abort the query can be very helpful in decision-support systems where resources must be conserved, and where a query that is not rewritten should not be allowed to run amuck inside the database.